It has long been a widely accepted practice to coat the metal surfaces with a resin layer as means for imparting corrosion resistance to the metal material. As coating methods used in this technology, there have been known a method of coating the metal surfaces with a solution obtained by dispersing a thermosetting resin such as an epoxy resin or a phenol resin in a solvent, and a method of sticking a film of the type of polyester, olefin or polyamide onto the metal substrate via a primer of the type of isocyanate, epoxy or phenol. According to these methods, however, some thermosetting resins and primers that are used contain a bisphenol A which may elute out. Besides, even from the standpoint of the necessity for coating the solvent and the cost thereof, it has been desired to provide means that substitutes for the above methods.
It has been widely known to utilize the heat-melting property of the thermoplastic resin for sticking the metal substrate and the thermoplastic resin together as represented by a method of sticking a pre-formed film of a thermoplastic polyester resin or the like onto a metal plate by heat-adhesion and a method of sticking a thin molten film of a thermoplastic polyester or the like that is extruded onto the metal plate.
The latter method of sticking onto the metal plate by the extrusion lamination offers such advantages that the treatment can be conducted at very high speeds involving decreased operation for forming films and decreased cost. When a generally employed polyester is used, however, the film swings and pulsates making it difficult to form a coating layer of a uniform thickness on the metal plate maintaining stability and close adhesion.
Further, the resin-coated metal plate used for producing cans requires close adhesion of the film during the working to withstand severe working such as draw working or draw-ironing of the resin-coated metal plate and, further, requires shock resistance (dent resistance) to withstand the vendor and the like. Depending upon the content, further, there are further required corrosion resistance and shock resistance to meet the cases of when placed under high-temperature and high-humidity heated conditions such as retort sterilization and to meet the cases of when stored in a hot vendor.
As the resin-coated metal plate having such properties, JP-A-2001-328208 proposes a resin-coated metal plate of which a thermoplastic resin layer comprises a polyester containing a polyethylene terephthalate as a chief component and an ethylene type polymer, the resin layer containing a tocopherol or a derivative thereof in an amount of 0.05 to 3% by weight.